Parse, the Y Combinator-backed startup that powers the back-end for mobile apps like Band of the Day and Hipmunk, is rolling out support for mobile web apps too.

The San Francisco startup has a new SDK for JavaScript that makes it easier to build HTML5 apps. Parse has long billed itself as the “Heroku for Mobile,” a service that takes over the hassle of building a scalable back-end for applications.

The new Javascript SDK comes with security controls, flexible data storage, advanced data queries, user authentication and geolocation. It’s built on top of the Backbone.js JavaScript framework from DocumentCloud. Parse co-founder Tikhon Bernstam said about 38 percent of the company’s developers had requested HTML5 support. Their current split between iOS and Android is about 65-to-35, but many use both.

“Developers want their users to be able to log into the same accounts with JavaScript mobile applications,” he said. “This actually makes a whole new class of apps possible.” The SDK works with mobile browsers including mobile Safari, the default Android browser and WebKit-powered browsers. It also works with desktop browsers like Chrome and Internet Explorer 8.

One of the things Parse is emphasizing most about the new SDK is its control over security. Parse says that the security enhancements in the SDK, like per-object Access Control Lists, have never been previously available in a JavaScript back-end service.

As for Parse itself, the company is growing at a 40 percent month-over-month rate and has about 16,000 apps on the platform. It’s a freemium service, with a $199 per month plan for apps with up to 15 million API requests per month. There is also an enterprise version that doesn’t have a listed price and is negotiable.

While there are many competitors including Kinvey and Stackmob, Bernstam says they’re not worried. “Our biggest competitor are developers who want to roll their own back-end solution.”